This invention relates to a method and apparatus for high temperature heating, melting, refining and superheating of materials, such as steel scrap, metals, ceramics or glass. The method and apparatus disclosed may be used as the major source of energy and also as an assisting energy source in melting furnaces, industrial heating and heat treating furnaces, kilns, incinerator and other high temperature applications.
Today, scrap preheating and melting is accomplished by different technologies, such as heat from the combustion of coke, oil or gas with air or oxygen, or from electric arc. Each of these technologies has some advantages and disadvantages. Using air from combustion has the advantage of being a low cost oxidizer, but, because air only contains 21% oxygen, it has the following disadvantages: low flame temperature, combustion instability inside cold scrap, low efficiency of heat utilization when scrap is hot due to escaping flue gases which waste about 50% of heat released by combustion.
The advantages of using essentially pure oxygen for combustion include: high flame temperature, good combustion stability, and a significant reduction of wasting heat with hot flue gases. The disadvantages of oxygen include its high cost and the necessity to cool the oxygen-fuel burner body.
The utilization of electrical energy is very expensive, but it provides a convenient means of operation and high product quality.
Four methods of oxygen introduction in the combustion process are used today:
injecting an oxygen stream into the fuel/air flame after the flame has left the fuel/air burner; PA1 injecting an oxy-fuel burner flame into a fuel/air burner flame after both flames have left their burners; PA1 enriching combustion air with oxygen by injecting oxygen in combustion air prior to supplying combustion air to the oxygen enriched air burner; and PA1 mixing of fuel, oxygen and air streams external to the burning device by lancing the three streams inside a hot furnace where said mixtures are burned.
The first two techniques of oxygen injection are recommended for increasing liquid and solid flame temperatures at glass melting furnaces or other high temperature furnaces where such fuel/air flames have sufficient dimensions and are located above the work being heated and are available for oxygen injection outside of the burner body. A high velocity oxy-fuel flame or an oxygen jet penetration into the core of a relatively cold oil or coal flame will superheat said core, therefore increasing the radiative heat flux of the micro-particles of carbon existing in such flame core without overheating the burner body. Oxygen enrichment of combustion air may be used for any fuel including hydrocarbon gases, particularly, natural gas. The oxygen enriched air burner has not found broad application for several reasons.
Burners for combusting fuel with air are old in the art, and burners for combusting fuel with pure oxygen (oxy-fuel) or oxygen enriched air are well known. However, the current state of the art burners do not operate satisfactorily across the full range of temperatures useful in high temperature heating, and do not allow for economical operation through control of flame chemistry, temperature, velocity and luminosity. Burners designed for use with hot air or oxygen enriched air typically use refractory tiles in the burners for continuous igniting of gases to stabilize the flame. However, due to the very high temperature of an oxy-fuel flame, refractory tiles cannot be used, and such burners are internally water or air cooled. The elimination of the burner tiles results in flame instability at lower temperatures and therefore limits the turn-down ratio of oxygen enriched air burners.
Another problem that often arises in oxy-fuel and oxygen enriched burners is the presence of excess oxygen in the flue gases. The hot furnace temperatures, together with the excess oxidizing ability of the flue gases, accelerate deterioration of expensive furnace components.
Also, in cases where natural gas is utilized as a fuel, an oxygen-fuel flame or an oxygen enriched air-fuel flame is not emissive. To be able to transfer heat, the flame would therefore have to touch the product being heated. This can create a problem with product distortion and oxidation.
The above mentioned technical, environmental and economical difficulties of oxygen enriched air burners are caused by the fact that using oxygen enriched air makes the combustion of fuel faster and less controllable inside of traditionally designed burning devices. These typically have a refractory lined combustion tunnel and use relatively lazy mixing techniques based on the low pressure of oxygen enriched air, the flow of which can be regulated by a traditional gas/air ratio regulator.